This invention relates to an optical information recording medium of write once (through a heat mode) type such as DVD-R, which is capable of performing a recording or reading data by means of a laser beam of a shorter wavelength region.
With respect to the means for recording and reading data such as images of character and graphic, picture or voice, an optical disc having a recording layer containing a pentamethine-based cyanine dye is known as a CD-R which is capable of recording and reading with a laser beam of 770 to 830 nm in wavelength. Recently however, DVD-R (a digital video disc-recordable, or a digital versatile disc-recordable), which is capable of recording and reading in high density with a red laser beam of 620 to 690 nm in wavelength for instance, which is shorter than the laser beam employed in the aforementioned CD-R, is now propagated as new media of the next generation.
As shown in Japanese Patent Unexamined Application H10-181211, the present applicants have proposed an optical information recording medium having a recording layer containing a specific kind of trimethine-based cyanine dye as being useful for such a DVD-R.
However, a trimethine-based cyanine dye having a benzene ring or a naphthalene ring, both being bonded to indole ring as shown in the aforementioned Japanese Patent Application, is accompanied with problems that when a substituent group in the benzene ring or naphthalene ring is hydrogen atom, etc. other than nitro group, the absorption spectrum of the recording layer having a dye layer containing such a dye becomes as shown in FIG. 1. Namely, the absorbance h.sub.2 of a second peak "b" representing an absorption due to the interaction and association between the dye molecules becomes too large in relative to the absorbance hl of the main peak "a" representing an absorption by the dye molecule itself which is based on the band gap energy in the dye molecule. As a result, the peak consisting of the main peak "a" and the second peak "b" fails to become sharp, and a half value width "d" of the spectrum (the width of the spectrum consisting of the main peak "a" and the second peak "b" in relative to the absorbance of h.sub.1 /2) which represents the degree of the sharpness of peak also fails to become sufficiently small.
If the spectrum fails to become sufficiently sharp as mentioned above, the recording sensitivity or the absorbance per unit film thickness of a recording layer at the occasion of forming pits by making use of the irradiation of laser beam onto a recording layer cannot be sufficiently increased. This invites not only the problems that the thickness of the dye layer is required to be increased, the recording power is required to be increased, or the recording speed is required to be decreased, but also the problems that a so-called heat interference (wherein the deformation of the configuration of the pits may be caused due to the accumulation of heat at the space between the pits) tends to be brought about at the occasion of recording, thus giving a bad influence to the characteristics of the recording layer such as modulation amplitude or jitter.